A number of methods are known to measure the deformation of a shaft, such as the shaft of a gas turbine or steam turbine or electric generator.
U.S. Pat. No. 4,995,257 discloses a shaft with two marks longitudinally spaced apart on it; light sources and detectors for the light reflected by the marks are connected to a circuit that calculate the angular deformation.
U.S. Pat. No. 4,347,748 discloses a shaft with a first and second reflecting surfaces at spaced apart positions; light sources and detectors for the light reflected by the reflecting surfaces are provided connected to a circuit that calculates the torque.
These known systems try to directly measure the shaft deformation (i.e. directly detect the deformation angle), it is clear that since the deformation is usually very small, in some cases this measure can be inaccurate.